Project Summary The ability to make accurate movements is vital to the many tasks we perform every day, and the loss of this ability results in a major decline in quality of life. Approximately 10 years ago, our lab developed a mathematical model of how the brain may update motor commands to maintain accuracy on a trial-by-trial basis. This model proposed that adaptation is the sum of two independent processes, one that learns significantly from error but has poor retention (fast process), and one that learns little from error but has strong retention (slow process). This two-state model matched behavioral data during motor adaptation and explained some unusual phenomena like spontaneous recovery. However, because the two states sum to produce a single motor output, it has been difficult to measure them directly. Here, we propose a novel method to simultaneously measure behavioral correlates of the putative fast and slow adaptation processes within a single movement using short-term saccadic adaptation. Conventional saccade adaptation paradigms induce learning by jumping the target along the axis of the motion of the eyes. The novelty in our paradigm is that the target step is perpendicular to the motion of the eyes, allowing for a clean dissociation of the primary motor commands from the commands related to adaptation. In many ways, cross-axis saccade adaptation replicates the fundamental design principles in reach adaptation in force fields. However, with saccades, there is the important advantage that the movement is too brief for within movement corrections from sensory feedback. In preliminary results, we show that this form of adaptation produces motor commands that can be cleanly dissociated into those that accelerate the eyes and those that decelerate the eyes. The acceleration component appears to learn weakly from error but shows robust retention. In contrast, motor commands that decelerate the eyes show strong learning from error but have weak retention. That is, a signature of the fast and slow processes may be present in the motor commands that move the eyes during a single saccade. We propose that this new framework provides a rich body of behavioral data that can help uncover the characteristics of the putative fast and slow processes in both reaching and saccades. The training plan for the proposed fellowship will follow an approach Dr. Shadmehr has developed for guiding students to tenure-track positions and successful scientific careers. This approach involves teaching the trainee to design insightful experiments, collect and analyze data, absorb the scientific literature, present scientific results, write papers, and describe future plans in funding proposals. Weekly, hour-long individual meetings between Dr. Shadmehr and the trainee will provide guidance and ensure appropriate progress. The proposed research and training will take place in the Shadmehr Lab at Johns Hopkins. This lab is part of the top-ranked Department of Biomedical Engineering and a very strong motor neuroscience group at Hopkins and is well-equipped for the study of human motor learning.